At its most basic level, QT is the theory that energy, and therefore most everything, comes in discrete packages, or "quanta", the same root from which we get our modern word "quantity". Like the ancient Greek idea of the "atom", these energy packages are believed to be indivisible. For example, the quanta for EM energy is the photon. Quantum theory states that an atom infused with enough energy to emit a single photon will do so, but that same atom, given slightly less energy, will not emit one-half of a photon.